Physics 2005 Jamb Examination

Adding impurities into a pure semiconductor is called doping. Doping introduces additional charge carriers (electrons in n-type or holes in p-type), which drastically increases conductivity.

Pure (intrinsic) semiconductors have limited conductivity because only a few electrons are excited across the band gap. After doping (extrinsic), donor or acceptor states make conduction much easier.

Thus, the conductivity increases. Correct option: B.

Silicon is a group IV element. To form an n-type semiconductor, it must be doped with a pentavalent (group V) element such as phosphorus, arsenic or antimony. The extra fifth electron becomes a free conduction electron.

If doped with trivalent (group III) elements, it forms a p-type semiconductor (holes as carriers). Therefore, correct group is V. Option A.

In gases, conduction occurs when atoms or molecules are ionized. This produces positive and negative ions, along with free electrons. Both ions and electrons act as charge carriers.

In contrast, in semiconductors conduction involves electrons and holes. Therefore, correct option: B (electrons and ions).

Cathode rays are streams of fast-moving electrons emitted from the cathode in a discharge tube. When they strike gas molecules or the glass wall, they cause fluorescence (glow).

Anode rays are positive ions moving in the opposite direction, and gamma rays are high energy electromagnetic waves, not relevant here. Correct: A, cathode ray.

Transformer equation: \(\tfrac{V_p}{V_s}=\tfrac{N_p}{N_s}.\)

Given \(V_p=240\,V, V_s=12\,V, N_p=4000\) turns. Then \(N_s=\tfrac{V_s}{V_p}\times N_p=\tfrac{12}{240}\times4000=\tfrac{1}{20}\times4000=200.\)

In a reverse-biased p–n junction, majority carriers are repelled away from the junction, so they do not contribute to current.

Only the few minority carriers (electrons in p-type, holes in n-type) are able to cross the junction and produce a small reverse saturation current.

Correct option: A (minority carriers).

Conductors allow electric current to flow freely due to the presence of free electrons. Sodium (Na), a metal, has delocalized electrons in its metallic bond, making it a good conductor.

Glass, wax, and plastic are insulators, which do not allow significant current flow. Correct option: C (Sodium).

A hygrometer measures humidity using the wet and dry bulb thermometer principle. The wet bulb cools by evaporation; the drier the air, the more evaporation, and the bigger the temperature difference.

Thus, a large difference indicates low relative humidity. Conversely, a small difference means high humidity. Correct option: D.

Density of mixture is given by:

\[ \rho = \dfrac{m_1 + m_2}{V_1 + V_2} \]

For W: \(m_1 = \rho_1 V_1 = 200 \times 3 = 600\,kg.\)

For L: \(m_2 = 150 \times 7 = 1050\,kg.\)

Total mass \(= 600 + 1050 = 1650\,kg.\)

Total volume \(= 3 + 7 = 10\,m^3.\)

Density of mixture \(= 1650 / 10 = 165\,kgm^{-3}.\)

Correct option: A.

At terminal velocity, forces balance:

Weight = Upthrust + Drag

So, Upthrust = Weight − Drag = 360 − 260 = 100 N.

Correct option: A (100 N).